Steam users who opted into the beta version of Batman: Arkham Knight managed to get hold of a patch yesterday that apparently fixed a lot of the problems the game had at launch. As of writing, however, the patch is no longer available.
Many of those in the forums who did manage to try the patch have said it makes the game run "butter smooth", with one reporting that it adds lots of graphics options and "even a bar to show how much Vram is being used". Another says they got constant 60fps, with possibly only a minor drop when driving.
Warner Brothers did say recently that we'd receive a new patch in the next few weeks, one that would fix the worst issues and expand the graphics options, so hopefully that's what these beta users managed to get their hands on and the rest of us should get it soon. Maybe then the game will be re-released, and then we'll get our hands on that Batgirl DLC.